Natural Killer cells (NK) are large granular lymphocytes of bone marrow origin that exhibit non-MHC restricted cytolytic activity against a variety of tumor and virally infected cells. The molecular basics of NK cell target recognition is relatively unknown. A receptor present on all NK cells and T cells that exhibit non-MHC restricted cytotoxicity has been identified and the gene for this receptor has been cloned. Previous work in our laboratory has shown that this receptor, 2B4, is able to activate cytolytic activity and cytokine secretion when conjugated by monoclonal antibody. However, it was recently shown that the gene for this receptor encodes two isoforms by an alternative splicing mechanism that differ solely in their cytoplasmic domains. These isoforms were shown to possess opposing functions in that one form functions as a stimulatory receptor and the other is inhibitory. Recently, the ligand for 2B4 has been identified as the CD48 molecule, which was previously thought to be the ligand for CD2. CD48 is expressed on all lymphoid cells and acts as a co- stimulatory molecule for B cells and Dendritic cells. However, the role of 2B4-CD48 interactions in the function of NK, T, B and dendritic cells as yet to be determined. In this proposal, we plan to characterize at a molecular level the signal transduction pathways involved in the function of both isoforms of the 2B4 receptor. We also plan to determine the ability of this receptor to communicate with other T or NK cell receptor signaling pathways. These studies will allow us to define the pathways that the 2B4 isoforms utilize in cellular communications and allow us to make predictions regarding the potential role of 2B4:CD48 interactions on immunological processes.